didn't need to do表示： 过去不必做某事, 事实上也没做。.

needn't have done表示： 过去不必做某事, 但事实上做了。

John went to the station with the car to meet Mary, so she didn't need to walk back home. 约翰开车去车站接玛丽，所以她不必步行回家了。

John went to the station with the car to meet Mary, so she needn't have walked back home. 约翰开车去车站接玛丽，所以她本不必步行回家了。 (Mary步行回家，没有遇上John的车。)

典型例题

There was plenty of time. She ___.

A. mustn't have hurried　 B. couldn't have hurried　　C. must not hurry　 D. needn't have hurried

答案D。needn't have done. 意为"本不必"，即已经做了某事，而时实际上不必要。

Mustn't have done 用法不正确，对过去发生的事情进行否定性推断应为couldn't have done, "不可能已经"。 must not do 不可以(用于一般现在时)。

英语六级语法知识：It is (high) time that

It is (high) time that 后面的从句谓语动词要用过去式或用should加动词原形，但should不可省略。

It is time that the children went to bed.

It is high time that the children should go to bed.

英语六级语法知识：比较if only与only if

only if表示"只有";if only则表示"如果……就好了"。If only也可用于陈述语气。

I wake up only if the alarm clock rings. 只有闹钟响了，我才会醒。

If only the alarm clock had rung.　　　当时闹钟响了，就好了。

If only he comes early.　　　　　　　但愿他早点回来。

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